1. Field of Invention
This invention relates to a projection-type display apparatus enlarging and projecting color light image onto a screen through a projection lens by decomposing the white light beam from a light source into three-color light beams of red, blue, and green, modulating these colored light beams through a light valves comprising liquid crystal panels according to given video information and recombining the modulated light beams of different colors after modulation. More specifically, the present invention relates to control driving liquid crystal light valves for such a projection-type display apparatus, or the gamma correction of input video signals that are supplied to the liquid crystal light valves.
2. Description of the Related Art
Fundamentally, a projection-type display apparatus comprises a light source, a color separator segregating white light beam emitted from a light source into three primary color light beams, three liquid crystal light valves modulating three color light beams, a light synthesizer synthesizing modulated color light beams and a projection lens enlarging and projecting synthesized, modulated light beam onto a screen.
(a) In general, the three liquid crystal light valves are matrix-type liquid crystal panels having a plural of pixels arranged in the matrix. The characteristic curve of transmissivity and applied voltage for a liquid crystal panel is irregular compared to the characteristic curve showing the relationship between the brightness and the applied voltage of a CRT. Therefore, it is necessary to apply a specific gamma correction to the input video signals, in order to obtain a appropriate projection image for a liquid crystal panel based on input video signals.
There are two types of gamma correction: the analog gamma correction in which input video signals is corrected corresponding to a preset gamma correction curve; and the digital gamma correction in which input video signal corrections are made based upon digital data of gamma correction pre-stored in ROM or other devices.
The former method, the analog gamma correction, in which corrections are applied according to an approximation curve, offers a more cost effective solution compared to the digital gamma correction. The drawback, however, is a lack of detailed correction capacity afforded by digital type gamma correction.
On the other hand, the digital gamma correction is well suited for implementing highly accurate corrections because of its ability to accommodate fine, multi-steps correction values. The drawback, however, is that it requires a large number of pre set correction values in curve-value ranges, thereby requiring significant memory overhead and processing throughput.
FIG. 29 shows the relationship between transmissivity of liquid crystal and applied voltage equivalent to level of video signal. Transmissivity from 0% to 100% are divided into 16 levels of gray scale. Voltage levels of video signal correspond to these gray scale levels. Voltage differences for generating each gray scale levels in the area B are substantially equal. On the other hand, voltage differences for generating each gray scale in the area A are sharply changed because of the uniqueness of the V-T (voltage transmissivity) curve.
Even under this circumstance, as an initial step, conventional digital gamma correction is applied to all range of applied voltage. In this case, however, voltage differences in the area A need more numbers of bit allocation than the area B. Applying a digital gamma correction to the area A requires a large amount of bit allocation. For this example, if digital gamma correction is performed with 256 bits, 100 bits, 40% of them should be allocated to the area A, corresponding only to 20% of all levels of gray scale. But, if a large amount of bits are allocated only to that area, the rest of bits which should be allocated to other area should be reduced. This reduced bits allocation causes coarse correction to affect undesirable accuracy in gamma correction.
Furthermore, input video signals can be of different types of signal format systems, such as the NTSC system or the PAL/SECAM system. Therefore, gamma corrections having fixed values are not capable of applying appropriate gamma corrections to input video signals of different formats.
(b) As described in Japanese laid-open application S62-145218 by the applicants of the present invention, liquid crystal is encapsulated between a transparent substrate and a counter substrate in a matrix-type liquid crystal panel for three liquid crystal light valves. Also, thin-film transistors and transparent pixel electrodes are formed in a matrix pattern on the transparent substrate. Common electrodes are formed on the counter substrate. In such a scheme, the transmissivity of the pixels is controlled by means of voltages that are selectively applied to the pixel electrodes.
As shown in FIG. 19, the white light beam emitted by white light source lamp 805 in the optical system is separated into three light beams of red, blue, and green. They enter liquid crystal light valves 925R, 925G, 925B, are modulated according to video information, and are recombined by prism unit 910, which is a color synthesis system.
Of the light beams of various colors that have passed through liquid crystal light valves 925R, 925G, and 925B, the green light beam G reaches projection lens unit 6 after passing through prism unit 910, whereas the red light beam R and the green light beam G are reflected at a right angle by the X-shaped reflecting surface of prism unit 910 before reaching projection lens unit 6.
Therefore, after passing through prism unit 910, the red light beam R and the blue light beam B that have passed through liquid crystal light valves 925R and 925B undergo a right-left optical image reversal relative to the green light beam G that has passed through liquid crystal light valve 925G.
Therefore, the conventional solution has been to ensure that the resulting optical image is oriented correctly, conventionally, while light valves 925R and 925B could be of the same structure, the remaining light valve, 925G, must be of an inverted structure in which the pixels have a reverse selection drive orientation.
Furthermore, conventional methods employ a drive voltage, for driving the liquid crystal light valves, that alternate in fixed cycles. The transmissivity of liquid crystal may change with the polarity of the applied voltage. Therefore, conventionally, as described in Japanese laid-open application S62-254124 by the applicants of the present invention, R and B liquid crystal light valves are driven in the same polarity (e.g., "positive") and the remaining G liquid crystal light valve with the inverse structure is driven in reverse polarity (e.g., "negative"), in order to smooth out the fluctuations and eliminates the problems. Thus, as shown in FIG. 28, the polarity (phase) of the alternating drive voltage for pixels in video information fields is reversed in the light valves for R and B aco light as compared to those for G light. The arrows in this Fig. show the scanning direction of driving signal for a given light valve.
Thus, as described above, a conventional projection-type display apparatus requires two different structures, namely two different shift registers of which scanning directions are reversed for the construction of three liquid crystal light valves.
(c) Further, in order to obtain a projection image that corresponds to input video signals using a matrix-type liquid crystal panel that composes three liquid crystal light valves, it is necessary to drive the liquid crystal light valves in correspondence with the input video signals. Input video signals is commonly based on the format signal system such as the NTSC system. In this system, as is well known, one frame is composed of two odd-and-even fields, such that the number of scanning lines per frame is 525. On the other hand, the PAL/SECAM system uses 625 scanning lines per frame, leaving 100 more vertical scanning lines for the NTSC system. Consequently, 100 scanning lines of video information can be lost from the video signals in case of NTSC system for liquid crystal projection display.
To avoid this problem, video signals are conventionally compressed in order to display a given image in its entirety. Specifically, a given set of video signals is culled to compress data so that the resulting image will have five-sixths the original number of video signals.
In this method, however, the conventional compression processing causes dropouts of some video signal lines. For example, as shown in FIG. 33, curved figures such as a true circle are rendered as discontinuous displays due to data compression.